1. Field of the Invention
The present invention relates to a method of preparing biomaterial for prosthetic use that has excellent mechanical properties and high calcification resistance. The method includes the processing of tissues with glutaraldehyde, by standard methods, to fix the tissue. The method then calls for oxidizing the tissue using photosensitive dyes or other means. The resulting tissue has the favorable mechanical properties associated with glutaraldehyde fixation and the resistance to calcification associated with oxidative fixation.
2. Description of the Related Art
Tissue transplantation is a rapidly growing therapeutic field as a result of improvements in surgical procedures, immuno-suppressive treatments, and increased knowledge of the graft-host interaction. Despite major advances, problems associated with tissue transplantation includes inflammation, degradation, calcification, and rejection of the transplanted tissue.
There are several applications for biomaterial tissue transplantation. Heart malfunction due to heart valve disorders can often be treated by surgically implanting a prosthetic valve. Mechanical and bioprosthetic heart valves, made from tanned tissue, are currently in use. Unfortunately, mechanical valves cause severe problems because of the water hammer effect (a transient pressure pulse associated with sudden changes in the velocity of a fluid), poor flexibility, and hemolysis of blood passing through the valve as it nears being fully closed. As a result of the hemodynamic characteristics of mechanical valves, patients are often on life-long anticoagulant therapy.
The limitations of mechanical valves lead to the use of bioprosthetic heart valves (A. Carpentier, J. Thorac. Cardiovasc. Surg., 58: 467, 1969). Tissue derived from porcine aortic valves or bovine pericardium are currently in use. These valves are better than mechanical valves because they have a shape and function similar to the valves they are replacing. In this manner, a centrally orientated blood flow path is maintained, the pressure drop across the valve is lowered, and hemolysis is greatly reduced.
Biomaterial must be stabilized prior to implantation into an animal different from the donor animal. This process of stabilization is known in the art as fixation or tanning. Collagenous biomaterial, usually the major component of a bioprosthesis, can be fixated by treating the material with aldehydes (Nimni et al., J. Biol. Chem. 243:1457-1466, 1968). Later, it was discovered that, of various aldehydes tested, glutaraldehyde best retards degeneration of collagenous tissue (Nimni et al., J. Biomed. Mater. Res. 21:741-771, 1987; Woodroof, E. A., J. Bioeng. 2:1, 1978).
Generally, the fixation process operates by blocking reactive molecules on the surface of and within the donor tissue, thereby rendering it substantially non-antigenic and suitable for implantation as well as crosslinking the collagenous matrix providing stability. Thus, the process of glutaraldehyde-fixation has been and continues to be applied to most varieties of experimental and clinical bioprostheses.
Early experimental and clinical studies of glutaraldehyde-preserved bioprostheses were of bioprosthetic heart valves. Degeneration of collagen and elastin were found to be major factors in the malfunction of bioprosthetic heart valves. Therefore, a method was developed for treating the biomaterial to inhibit inflammatory reactions by host cells while enhancing strength and flexibility, and to prevent the degeneration of collagen and elastin (A. Carpentier, Biological Tissue in Heart Valve Replacement, M. I. Ionescue et al. (Eds.), Butterworth, London, 1972). The method for treating tissues involved washing the tissue in acid, e.g. Hanks solution, and then oxidizing mucopolysaccharide and glycoprotein with metaperiodate to form aldehyde groups, and finally binding and crosslinking the aldehyde groups with amines. The cross-linkages were then stabilized with sodium borohydride.
The data compiled from these early studies demonstrated the excellent biomechanical properties, high resistance to enzymatic degradation, excellent hemodynamic properties and minimal thrombogenicity of the glutaraldehyde-preserved heart valve. However, these bioprostheses may develop failures due to tissue degeneration or, particularly, calcification. Calcification, which causes prosthesis degeneration, is an especially significant disadvantage to the use of tissue-derived prostheses. Indeed, cuspal calcification, i.e. calcification of the bicuspid tissue, accounts for over 60 percent of the failures of cardiac bioprosthetic valve implants, such failures being substantially more frequent in children than in adults.
Calcific deposits in either porcine valves or bovine pericardial valves often nucleate in cell membranes, cell nuclei, and intracellular organelles within 48 hours of transplantation. These deposits increase in size and number over time. The deposits often destroy cells, cleave collagen bundles, and form nodules associated with clinical failure of the tissue (F. J. Schoen et al., in Surgery For Heart Valve Disease: The Proceedings of the 1989 Symposium, E. Bodnar, Ed., 679-85, 1990).
Several methods have been developed to reduce the tendency of the tissue to calcify, but the results are inconclusive. Calcium crystal inhibitors such as phosphonate salts have been put into the tissue (R. J. Levy et al., Circulation, 71: 349, 1985). Detergents such as sodium dodecyl sulfate placed into the tissue inhibit the onset of intrinsic calcium deposition in glutaraldehyde fixed xenograft tissue (D. J. Lentz et al., in Cardiac Biotissue Grafts; Proceedings of the Second International Symposium, L. W. Cohn and V. Galluci, Eds. New York: Yorke Medical Books, 306-19, 1982). Another method of inhibiting calcification is by pretreating the tissue with aluminum ions (C. L. Webb et al., TASAIO, 34: 855,1988), or ferrous ions (M. Baldwin et al., Trans. Soc. Biomat., 14: 61, 1991). Another method of inhibiting calcification is introducing anionic polysaccharides such as chondroitan sulfate (G. M. Bernacca et al., Biomaterials, 13: 345, 1992), or even aspirin to the tissue (U.S. Pat. No. 4,838,888). Yet another method is to covalently bond sulfonated polyethylene oxide to the tissue (U.S. Pat. No.5,697,972). Moreover, treatment with alpha amino oleic acid may prevent calcification of glutaraldehyde treated bioprosthetic heart valves, but problems with tissue degradation have been reported.
Inactivation of residual glutaraldehyde with an amino compound such as chitosan or glycine--gentamicin prevents calcification in adult rats but not in adolescent rats (J. Chanda, Ann. Thorac. Surg., 60:S339-42, 1994). If the concentration of glutaraldehyde used to treat the tissue does not exceed 0.25%, then subsequent treatment of glutaraldehyde-glycine-gentamican tanned tissue followed by treatment with partially degraded heparin improves resistance to calcification (U.S. Pat. No. 5,645,587). Other methods of treating glutaraldehyde-fixed tissue include soaking the tissue in a polyol such as propylene glycol, 1,3-propanediol, or glycerol (U.S. Pat. No. 5,476,516). The long term stability of these treatments is not known.
It has been found that glutaraldehyde, one of the most popular fixing agents in terms of the mechanical properties of the tissue, enhances susceptibility to calcification. The extent of glutaraldehyde cross-linking is clearly important, although the specific mechanism is not known (C. L. Webb et al., Ann. Thorac. Surg., 60:S359-64,1995). The slow release of residual glutaraldehyde from the prosthesis reinforces host plasma-bound calcium complexes.
Substitutes for glutaraldehyde fixation have been proposed. Efforts have been made to use alternative fixing agents such as carbodiimide in place of glutaraldehyde (T. Okoshi et al., TASAIO, 36; 411, 1990). Cross-linking has been achieved using suberic acid, a di-carboxylic acid, and 1,6-hexane diamine, thereby forming amide linkages (U.S. Pat. No. 5,447,536). Fixing by soaking the tissue in an aqueous solution of photo-oxidative catalyst or compound, followed by exposure to light, thereby fixing the tissue by photo-oxidation, has also been used (M. A. Moore et al., J. Biomed. Mater. Res., 28:611-18, 1994).
Currently, most bioprosthetic tissue is fixed via treatment with glutaraldehyde. Glutaraldehyde treatment provides a tough, durable tissue with mechanical properties often superior to those of alternative methods of fixation. Unfortunately, as described above, calcification of the tissue often results, which leads to failure of the tissue in the patient. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.